The 18,000 km2 expanse of the Semipalatinsk Test Site (indicated in red), which covers an area the size of Wales. The Soviet Union conducted 456 nuclear tests at Semipalatinsk from 1949 until 1989 with little regard for their effect on the local people or environment. The full impact of radiation exposure was hidden for many years by Soviet authorities and has only come to light since the test site closed in 1991.[3]

2007 ISO radioactivity danger symbol. The red background is intended to convey urgent danger, and the sign is intended to be used in long-term radioactive waste repositories, which might survive into a distant future where other danger symbols may be forgotten or misinterpreted.

Radioactive contamination or radiological contamination is the contamination of the environment with radioactive materials, where these materials are not present. Many radioactive substances have very long half lives; this means that if they are present in the envioroment, they can be dangerous for a very long time. Many nuclear power plants produce such substances; usually they are processed to radioactive waste. There is little danger that comes from radioactive waste,if it is stored safely.